Inlet air filtration systems are generally employed for use with gas turbines and operate by removing salt, dust, corrosives and water (hereinafter referred to as “foreign matter”) from inlet air in order to prevent their entry into the gas turbine. Foreign matter can enter the gas turbines in various forms, such as solids (i.e., dry salt) or aqueous solutions (i.e., wet salt), and corrode the gas turbine elements. This corrosion could lead to operational inefficiencies or failures and financial losses. As such, it is typically necessary to provide for corrosion mitigation to the gas turbine engine by way of an inlet filtration system that reduces the amount of corrosives entering the gas turbine.
The corrosives can exist in several states which can enter the gas turbine. The first state includes Solid Particulate Corrosive elements. These include salt and oxide particles which can be removed by high efficiency filters. The second state includes Liquid, or, rather, Aqueous Corrosives elements. These include aqueous chlorides or acids, the removal of which cannot generally be efficiently accomplished by particulate filters.
In both cases, the corrosives can be moved along airflow by, typically, two main transfer mechanisms. These include solid salts deposited on particulate filters that can deliquesce when the humidity of ambient air rises beyond about 60% relative humidity (RH) or when filter elements get wet and salts, which are dissolved via rain, fog, mist and other sources of water, enter the inlet air stream. Once salt solutions pass the final filters, there is a potential for the liquid to dry and for salt to precipitate out of solution. This salt precipitate or crystallized salt can now enter the gas turbine.
Current filtration systems available on the market specifically for salt and water removal are generally classified into 3-stage systems and barrier-type systems. The 3-stage systems include a first vane/moisture separator, as a first stage, coalescing filters, as a second stage, and a second vane separator or a water tight high efficiency filter, as a third stage. The coalescing filter captures small salt aerosol droplets and causes them to coalesce into larger droplets and which can then be drained off as salt water. The coalescing filter also removes dust and dry salt particles from the inlet air which may be less than 1 micron in diameter and hydroscopic. The third stage removes any remaining droplets from the airstream, such as droplets that form from dry salt particles filtered by the coalescing filter, which take on water from humid inlet air and which are re-released into the airstream.
In a relatively dry environment in which the 3-stage system is used, a vane separator can be used as final stage, and dry salt particles may accumulate on the rear of the coalescing filter. These dry salt particles can then take on water from the humid inlet air and be re-released into the airstream as droplets that are not large enough to be removed by the second vane separator but which can lead to salt accumulation on the gas turbine elements.
In the barrier-type systems, there are typically two stages of filtration. The first stage is a coalescing pre-filter, which captures and coalesces droplets from the airflow. A large portion of the water drains away, but some is re-released into the airstream. The second, final stage comprises a watertight high efficiency filter, which is watertight and allows air, but not water, to pass through. This captures both dry and wet salt and fine particulate. There is no third stage vane separator within this system.
In practice, barrier-type systems rely on a 100% effectiveness of the filter frame and media seal to prevent salt water proceeding to the gas turbine. This is achievable on new and clean filtration systems on small gas turbines with few filters, but requires maintenance to keep it working properly and becomes impractical to scale on larger machines. Therefore, while the barrier-type system can be effective at stopping the migration of salt toward the gas turbine elements, the primary failure mode is seen as being the sealing mechanism, if installation and maintenance is carried out incorrectly.